As an important styrene-based thermoplastic elastomer (TPS), poly(styrene-b-butadiene-b-styrene) (SBS) is extremely important in industrial applications, including bitumen modification, adhesives, shoe manufacturing, polymer toughening and other aspects. SBS can be plasticized and molded at a high temperature and shows characteristics of rubber elastomer at room temperature. Polystyrene block and polybutadiene block are thermodynamically incompatible, and thus there is a micro-phase separation therebetween, polystyrene block microdomains dispersed in polybutadiene block continuous phase can act as physical cross-linking points at room temperature, the microdomains will disappear when heated to above the melting temperature of polystyrene. Therefore, SBS material can be processed, for example by injecting, blow molding, extruding, compression molding and the like.
In the existing inventions of industrial technologies, poly(styrene-b-butadiene-b-styrene) block copolymers are prepared using anionic solution polymerization process. In the anionic solution polymerization method, since chain termination and chain transfer reactions do not happen, polymers with predetermined molecular weight and narrow distribution can be prepared; and meanwhile, microstructure of the block copolymer, such as composition, molecular weight and its distribution of blocks, sequence of blocks, can be controlled by regulating the polymerization temperature, the initiator system, the order of addition of monomers, solvent or coupling agent and the like. However, due to using organic metal as a catalyst, the polymerization condition is very severe and most of solvent needs to be recycled. Compared with radical polymerization, the anionic solution polymerization has huge disadvantages in terms of energy consumption and environmental protection, as well as polymerization conditions. Further, under mild conditions, the anionic solution polymerization process cannot introduce monomers with polar groups, which limits the application of SBS in many fields.
In 1998, Graeme Moad, Ezio Rizzardo et al. found the reversible addition fragmentation chain transfer (RAFT) radical polymerization, after that, people have been exploring the technology of preparing block polymers through radical polymerization. The chain transfer agents used in this technology is referred to as reversible addition fragmentation chain transfer agent. As the reversible addition fragmentation chain transfer is suitable for a wide range of monomers, has radical isolation effect in a heterogeneous system and has rapid reaction speed, it is currently considered as one of the most promising radical polymerization technologies for industrialization. The Reversible addition fragmentation chain transfer technology can very effectively control the polymerization of monomers, and the molecular weights in a large range can be controlled to achieve a target molecular weight and a narrow molecular weight distribution. Radical polymerization can be carried out by emulsion polymerization. The emulsion polymerization is characterized by low viscosity and having no organic solvent, etc., and thus can be directly used to prepare polymer latex. The latex can be directly applied to paints, adhesives and other fields, and thereby brings great convenience for industrial production. In living radical polymerization, most of polymer chains can remain active during polymerization, and thus multi-block copolymer can be prepared by adding different monomers in different steps.
The common problems existed in RAFT emulsion polymerization are emulsion instability, molecular weight runaway, broad molecular weight distribution and the like. Gilbert et al. solved the problem of emulsion instability by using an amphiphilic macromolecule polymethacrylic acid-polybutyl acrylate as the reversible addition fragmentation chain transfer agent to conduct RAFT semi-continuous emulsion polymerization of styrene fed with starvation method, but this process is complex, and the obtained molecular weight has a great deviation from the predetermined molecular weight, that is, the target block copolymer was not obtained. In 2008, Charleux et al. conducted a batch emulsion polymerization of styrene by using polyethylene oxide-containing macromolecule as the reversible addition fragmentation chain transfer agent, with final conversion of only 66.7% after 22.7 hours. Other amphiphilic macromolecule reversible addition fragmentation chain transfer agents, such as polystyrene-polyvinylphenyltriethyl ammonium chloride as a two-block reversible addition fragmentation chain transfer agent, polydiethylaminoethyl methacrylate as a mono-block reversible addition fragmentation chain transfer agent, polyethylene oxide-polydiethylaminoethyl methacrylate as a two-block reversible addition fragmentation chain transfer agent, do not exhibit controllability of molecular weight and molecular weight distribution in batch emulsion polymerization of styrene. At present, the reason for the failure of RAFT emulsion polymerization of styrene is that polyacrylic acid-based amphiphilic macromolecule reversible addition fragmentation chain transfer agents have inappropriate length ratios of the hydrophilic segment to the lipophilic segment and thus these agents will be dissolved into water to form an aqueous phase only after additionally adding an alkali thereinto, and the formed aqueous phase has a pH≧5.5, leading to such results of polymerization reaction that the molecular weight of the product is runaway, molecular weight distribution is broad, the polymerization inhibition period is very long, the reaction speed is slow, the final conversion is low, the emulsion is unstable, and high molecular weight polymers and block copolymers cannot be well synthesized. Yingwu Luo et al. found that the amphiphilic RAFT agents can be dissolved into water without neutralization, by extending the length of ionizable polyacrylic acid segments of amphiphilic RAFT agents, and a supplementary addition of alkaline solution during emulsion polymerization will lead to ionization of the carboxyl of the hydrophilic segments of the amphiphilic macromolecule reversible addition fragmentation chain transfer agents, to increase the electrical charges and improve the stability of the emulsion, and thereby an amphiphilic block polymer with high molecular weight has been successfully prepared, where the hydrophilic monomer may be acrylic acid or methacrylic acid, and the lipophilic monomer may be styrene or acrylic esters. This process has short reaction time and high final conversion, and also the actual molecular weight is consistent with the predetermined molecular weight, the molecular weight distribution is <2.5, etc. Poly(styrene-b-butyl acrylate-b-styrene) as a three-block copolymer can be prepared according to this process.